The page you refer to would have been created dynamically, that is, it was a one-off page exclusive to you - if it's not in your browser history there will be no copy of it anywhere.
That said, did you make a payment at the time? Even if it was a deposit, the amount would perhaps have reflected the promotion in some way.
Failing that, all you can do is negotiate with the hotel - it may not want to lose the booking altogether over something like this. Tell them that you made this booking in good faith, and that you relied upon the information you were shown in the confirmation page being correct. If all else fails, there is always Twitter - holiday hotels don't like negative reviews on social media.